Continuous digester having selfregulating wood chip level

ABSTRACT

A CONTINUOUS UPFLOW DIGESTER OF THE TYPE WHEREIN LIQUID IS ADVANCED UPWARDLY THROUGH A COMPACTED MASS OF WOOD CHIPS IN A PROPELLING ZONE TO CREATE A FORCE FOR PROPELLING UPWARDLY THE COLUMNAR MASS OF WOOD CHIPS WITHIN THE VESSEL, SUCH VESSEL HAVING A TAPERED PORTION ADJACENT ITS LOWER END, THE LOWER END OF THE CHIP MASS NORMALLY BEING MAINTAINED THEREWITHIN IN A SELF-REGULATING MANNER WHILE WOOD CHIPS ARE ACCRETED THEREON AT RELATIVELY LOW LIQUID FLOW RATES BY REASON OF THE PROGRESSIVELY UPWARDLY DECREASING VELOCITY OF PROPELLING LIQUID CREATED WITHIN SAID TAPERED VESSEL PORTION AND INCREASING CHIP MASS LIFTING FORCE CREATED UPON DOWNWARD MOVEMENT OF THE LOWER END OF THE CHIP MASS.

March 27, 1973 K. E. VOGEL 3,723,243

CONTINUOUS DIGESTER HAVING SELF-REGULATING WOOD CHIP LEVEL Original Filed May 18, 1967 I 24 v l l DRAINAGE ZONE L J U COUNTER CURRENT i WASHING ZONE COOKING AND 1% HEATING ZONE 44 IMPREGNA T/ON AND HEATING ZONE V PROPULSION ZONE United States Patent Ser. No. 190,245 Int. Cl. D21c 3/.00

US. Cl. 162-52 3 Claims ABSTRACT OF THE DISCLOSURE A continuous upfiow digester of the type wherein liquid is advanced upwardly through a compacted mass of wood chips in a propelling zone to create a force for propelling upwardly the columnar mass of wood chips within the vessel, such vessel having a tapered portion adjacent its lower end, the lower end of the chip mass normally being maintained therewithin in a self-regulating manner while wood chips are accreted thereon at relatively low liquid flow rates by reason of the progressively upwardly decreasing velocity of propelling liquid created within said tapered vessel portion and increasing chip mass lifting force created upon downward movement of the lower end of the chip mass.

This is a continuation of application Ser. No. 43,559 filed June 4, 1970, which is a continuation of application Ser. No. 639,336, filed May 18, 1967, both of which are now abandoned.

This invention relates to the continuous digesting of wood chips in an upflow digester, and more particularly to novel apparatus and methods for controlling the continuous advance of a compacted mass of wood chips upwardly throughout the length of such a digester.

In upflow digesters of the type wherein liquid is advanced upwardly through a propelling zone in the lower portion of the submerged columnar mass of compacted wood chips at a liquid flow rate effective to create a force which will propel upwardly the entire columnar mass of wood chips extending throughout the length of the digester, it is necessary to control the position of the bottom level of the chip mass at least in an upward direction in order to maintain a propelling zone of adequate length. It is also desirable to control the position of said lower end in a downward direction to space it above the lower end of the digester so that wood chips may be fed freely into the digester for acccretion onto sad lower end,

Heretofore, it has been known that such control can be accomplished by sensing the position of the bottom end of the chip mass, as by mechanical devices, similar to that shown in U.S. Pat. No. 2,878,116, but with its resulting mechanical complication, or by suitable radiation devices. Nevertheless, such known expedients are deficient in one or more respects, especially in large digesters operating at high temperatures and pressures, as practical radiation devices are not effective because of the long measurement paths and mechanical devices by reason of the necessary stuffing boxes and the like.

It is a major object of this invention to provide in such digesters means for controlling in a self-regulating manner the position of the lower end of the wood chip mass.

3,723,243 Patented Mar. 27, 1973 It is another major object of the invention to provide in such digesters means for moving wood chips up to the bottom of the propelling zone of the chip mass at relatively low liquid flow rates.

In general, the present invention accomplishes such objects by providing a vessel having a tapered portion adjacent its lower end, the lower end of the chip mass normally being maintained therewithin in a self-regulating manner while wood chips are accreted thereon at relatively low liquid flow rates by reason of the progressively upwardly decreasing velocity of propelling liquid created within said tapered vessel portion and increasing chip mass lifting force created upon downward movement of the lower end of the chip mass.

For the purpose of fully explaining the above and further objects and features of the invention, reference is now made to the following detailed description of a preferred embodiment thereof, taken together with the accompanying drawing showing a diagrammatic side elevation, partly in section, of apparatus accordinig to the invention.

Referring to the drawing, the pressurized vessel 12 therein shown is generally cylindrical except for its lower tapered end portion 13 and is, for example, of the order of ten feet or more in diameter and one hundred feet in length. A wood chip-liquid infeed is provided at the lower entrance end of the vessel 12 in the form of a tangential inlet 16 to which is fed, through pipe 17, by means of a pump 18, which may be of the type shown and described in Pat. No. 2,908,226, a wood chip liquid mixture from a supply tank 20. At the upper end of the vessel 12 is provided a discharger for the pulped wood chips in the form of a scraper 22 which discharges drained pulped wood chips through discharge outlet 24 to discharge mechanism at the lower end of a down-flop pipe (not shown) such as is described in Pat. No. 3,206,356.

Preferably, within vessel 12, generally along the central vertical axis thereof, are provided a series of three concentric injection pipes 41, 43, and 53 having outlets 42, 44 and 54, respectively, at successively higher levels. Also within vessel 12 are provided strainers 46 and 48, together with said outlets establishing a plurality of zones within the vessel by advancing liquid for either co-current or countercurrent flow within said zones as desired. For example, there is provided an impregnation and heating zone with superposed heating and cooking zones with co-current and countercurrent flow and a countercurrent washing zone with an uppermost draining zone, the liquid flows being shown by the single arrows and the wood chip movement by the double arrows. A liquid level control 50 is provided near the upper end of vessel 12 to establish the liquid level below scraper 22 as by operating valve 52 in upper strainer 48 to provide the chip drainage zone.

A propelling liquid system is provided for advancing liquid upwardly through a lower portion only of vessel 12 for propelling the entire columnar chip mass upwardly through the vessel by high velocity liquid flow upwardly through said lower portion. Broadly, the apparatus components of such system include a lower propelling liquid strainer 30 spaced a substantial distance above vessel inlet 16 and below scraper 22 defining a propelling zone therebelow and a recirculation pump 32, together with connecting pipes 34 and 38 and valve 55 for controlling recirculation of liquid to pump 18.

More specifically as to the operation of the above described apparatus, it has been established in the commercial operation of digesters of the type shown, that the wood chips are present therein in the form of a columnar mass of contacting and compacted but discrete wood chips through which liquid can flow more or less independently of the wood chips in the mass but subject to a flow resistance due to the presence of the wood chips. Because the wood chips are present in the mass in discrete form but in contact with one another, the wood chips within the mass can be advanced upwardly throughout the entire length of the columnar mass maintained within the vessel by advancing liquid upwardly through a lowermost portion only of the chip mass at a suitably high liquid flow rate. However, in order to do so, it is desirable to maintain the lower end M of the chip mass in a position spaced above the lower end of the vessel, in order that the clogging and jamming of wood chips within inlet pipe 17 may be avoided.

According to the present invention, the lower tapered vessel portion 13 of upwardly increasing cross-sectional area is utilized, in conjunction with the upward flow of propelling liquid, normally to maintain the lower end M of the columnar mass of wood chips within the tapered vessel portion, suitably positioned below the propelling liquid strainer 30 and above vessel inlet 16. This is made possible both because of the increase in lifting force occurring as a result of resistance to fluid flow within the compacted chip mass when the propulsion zone length is increased, and the velocity change with distance which occurs throughout the tapered vessel portion 13, such being greatest at liquid inlet 16 and progressively decreasing upwardly as the vessel cross section increases.

More specifically, in the development of upfiow digesters of the type wherein liquid is advanced upwardly through a propelling zone to create a force which will propel upwardly a columnar mass of wood chips within the digester, it was discovered that under some conditions, in a vessel of essentially uniform cross-sectional area, effective chip mass propelling forces could be created at liquid flow velocities so low as to be incapable of moving free wood chips upwardly through the liquid column below the chip mass. Under such conditions, then, the minimum practical propelling liquid flow velocities were established, not by the propelling force needed, but by the flow velocity required to lift the wood chips through the liquid column up to the bottom of the chip mass. This was not only wasteful of pumping capacity, but sometimes created higher chip mass propelling forces than were desirable.

By applying the principles of the present invention, however, I have found that suitably high fluid flow velocities are readily created in the tapered liquid column at the inlet end of the vessel so that the free wood chips are lifted away from the lower end thereof, yet almost any desired low velocity may be created and maintained within the tapered columnar chip mass for propelling it. In addition, my novel use of a tapered lower vessel portion unexpectedly results in the self-regulating control of the lower end of the chip mass to an adequately uniform predetermined level within the tapered vessel portion. This apparently occurs because of the large pressure changes which take place within the lower end of the tapered columnar wood chip mass as a result of increasing liquid velocities caused by decreasing vessel cross section area. Thus, as the individual wood chips accrete on the bottom of the chip mass, its level M will move downwardly. The added increment will create increasingly higher propelling forces due to the higher liquid velocities through it as the vessel cross section decreases. This has been found to result, in a fairly short time interval, of the movement of the entire chip mass upwardly, whereupon the process repeats itself. Level control of the lower end M of the chip mass is thus achieved without sensors and their control devices.

Both the taper and the length of the propelling zone in relation to the diameter or other transverse dimension of the chip mass is important in maintaining flow uniformity and in preventing channelling, that is, the opening of large liquid flow channels through the chip mass, as tends to occur between closely spaced fluid inlets and outlets.

It is preferred that the length of the propelling zone be of at least the same order or somewhat greater than that of the transverse dimension of the chip mass, but certainly not less than about one-half of said dimension, as with short propelling zones, the necessary liquid flow velocity tends to become unduly high. It is also preferred that the vessel taper to a cross-sectional area at the bottom of the propulsion zone of at most about one half to one third that of its maximum cross-sectional area at the upper end of the propelling zone at propelling liquid strainer 30.

To operate the digester of the drawings according to the present invention, it is simply necessary to provide generally axial liquid upflow, preferably turbulent through the wood chips in the propelling zone, at the lower end of the chip mass and the underlying tapered liquid column, preferably by recirculation through the propelling zone and column. The position of the lower end of the compacted chip mass is self-regulated as above described at a level determined by adjusting the recirculation by control valve 55, to establish a desired predetermined level which is thereafter maintained in a selfregulating manner as above described. Otherwise, the digester operates in a conventional manner, with uncooked chips entering through tangential inlet 16 and being discharged at outlet 24, the other liquid flows within the digester being at conventional values as may be needed for treatment of the mass of wood chips within the vessel.

Various modifications of the invention Within the spirit thereof and the scope of the appended claims will be apparent to those skilled in the art.

What is claimed is:

1. In a method for the continuous pulping of wood chips in an upright elongated reaction vessel by feeding substantially continuously said wood chips and a liquid into said reaction vessel adjacent the lower end thereof to maintain therein a columnar mass of compacted wood chips with the lower end thereof normally spaced upwardly from the lower end of said vessel with a liquid column therebelow and advancing the wood chips upwardly throughout the length of said column by advancing propelling liquid upwardly through a liquid propelling zone in the lower portion of said column above the lower end thereof,

that improvement which consists of:

establishing a progressively upwardly increasing cross-sectional area of a portion of said mass and said liquid column adjacent the lower end of said vessel beneath a portion of said mass and said liquid column of generally uniform cross section thereabove, and

maintaining the lower end of said mass normally adjacent a predetermined level within said tapered portion solely by the upwardly progressively decreasing velocity of propelling liquid flow within said tapered mass and liquid column.

2. In a method as claimed in claim 1 wherein said mass and said liquid column within said tapered portion, taper to about half of the cross-sectional area of said mass at the upper end of said propelling zone with said generally uniform cross section portion thereabove.

3. In a method for the continuous pulping of wood chips in an upright elongated reaction vessel by feeding substantially continuously said wood chips and a liquid into said reaction vessel adjacent the lower end thereof to maintain therein a columnar mass of compacted wood chips with the lower end thereof normally spaced upwardly from the lower end of said vessel with a liquid column therebelow and advancing the wood chips upwardly throughout the length of said column by advancing propelling liquid upwardly through a liquid propelling zone in the lower portion of said column above the lower end thereof,

that improvement which consists of:

establishing a progressively upwardly increasing cross-sectional area of a portion of said mass and said liquid column adjacent the lower end of said vessel beneath a portion of said mass and said liquid column of generally uniform cross section thereabove, the length of said mass and said liquid column within said tapered portion being at least about half of the transverse dimension of the generally uniform cross section of said mass thereabove and tapering to less than about half of the cross-sectional area of said generally uniform cross section maintaining the lower end of said mass normally maintained adjacent a predetermined level within said tapered portion solely by the upwardly progressively decreasing velocity of propelling liquid flow rate at uniform quantity within said tapered mass and liquid column.

References Cited 5 UNITED STATES PATENTS 2,383,684 8/ 1945 Richter 1'62237 X 2,878,116 3/1959 Calrsmith et a1 162237 3,397,110 8/1968 Rosenblad 16252 X 3,467,573 9/1969 Vogel 162-237 X 10 3,579,417 5/1971 Carlsmith 16217 FOREIGN PATENTS 60,797 12/ 1922 Sweden l62-52 US. Cl. X.R. 

